This invention relates to improved cut orientations and dimensions for lead based single crystal compositions including Pb(Mg1/3Nb2/3)O3xe2x80x94PbTiO3 (xe2x80x9cPMNxe2x80x94PTxe2x80x9d), Pb(Zn1/3Nb2/3)O3xe2x80x94PbTiO3 (xe2x80x9cPZNxe2x80x94PTxe2x80x9d), and Pb(Sc1/3Nb2/3)O3xe2x80x94PbTiO3 (xe2x80x9cPSNxe2x80x94PTxe2x80x9d), type relaxor single crystal compositions. Specifically, improved electromechanical properties of single crystals were obtained by orienting the crystal""s thickness and width along certain directions. In addition, improved properties were also obtained by preparing single crystals of specific dimensions. These combinations of cut orientations and dimensions give rise to improved crystal processibility and excellent electromechanical properties.
A transducer is a device that converts one form of energy to another. For example, ultrasonic transducers convert electrical energy to mechanical energy and vice versa. An ultrasonic transducer includes an ultrasonic transmitting/receiving element(s) typically consisting of piezoelectric element(s) connected to electrodes. The electrical energy supplied to the electrodes electrically excites the ultrasonic probe element(s) causing them to vibrate at a given frequency. The vibrations then give rise to acoustic waves (in this case, ultrasonic waves) which, upon impinging on an interface representing a junction between two media, are either reflected or transmitted. The reflected waves can be detected by the same piezoelectric probe. This reflection and transmission of acoustic waves at the interface between two media is the basis of ultrasonic imaging. An ultrasonic imaging apparatus incorporating this ultrasonic probe has been used to examine the interior of a human body or to detect flaws in a metal welded portion.
B-mode imaging, color flow mapping (CFM), and Doppler are the common ultrasonic diagnostic imaging methods used on human bodies. CFM is capable of two-dimensionally displaying in color the blood flow velocity in organs such as the heart, liver, kidney, spleen or carotid artery by using a Doppler shift of ultrasonic waves caused by the bloodstream, as well as displaying tomographic images (the so-called B-mode images where the echo signals are represented as intensity-modulated lines in a display) of human bodies. Diagnostic capability has been dramatically improved by these medical diagnostic methods.
A commonly used ultrasonic probe configuration comprises an array of a few tens to about 300 ultrasonic transmitting/receiving elements each of which is made of a strip of piezoelectric material. With this configuration, it is difficult to obtain matching with a transmitting/receiving circuit because the impedance of each piezoelectric element increases as the number of ultrasonic transmitting/receiving elements increases. Also, for certain applications, the surface of a phased-array probe has to be kept as small as possible if, for example, the probe used to image the heart through rib spacing or used internally on a live subject.
Ultrasound as an imaging method has tradeoffs. When high frequencies are used, the resolution is improved but the penetration is reduced. Thus, in many cases, more than one transducer is needed to perform a diagnosis because of the necessary penetration depth and resolution. But good penetration and resolution cannot be obtained at the same time. The human tissue has strong non-linear characteristics. When it is imaged by an ultrasonic signals, it generates harmonic signals, such as first, second, and third harmonic signals. Recently, with the advent of tissue harmonic imaging, it is possible to increase penetration by transmitting at a lower fundamental frequency (f0) and at the same time, to increase resolution by detecting the second harmonic signal (2f0) arising from the nonlinear response of the subject.
Another ultrasound application utilizing harmonics is the contrast harmonic imaging. In this type of imaging, the contrast agents used are typically gas-filled microspheres (bubbles) which resonate at certain ultrasonic frequencies. When the contrast agents are insonified at one frequency, they generate large harmonic signals due to the contrast agents"" nonlinear response. The use of contrast agent significantly improves the detection of blood-filled structures and blood flow velocity in the arterial systems.
For harmonic imaging, broadband transducers must be used to both transmit and received a bandwidth wide enough to encompass the fundamental and harmonic frequencies. Because current PZT-type transducers do not fulfill this bandwidth requirement, their performance in harmonic imaging is lower than transducers that meet the requisite bandwidth.
Nonetheless, piezoelectric materials such as PZT based ceramics are widely used for medical ultrasound transducers. Two of the important criteria for choosing a piezoelectric material for ultrasound transducer applications are high values of the longitudinal coupling constant (k33) and dielectric constant (K). A high coupling constant is desirable because it represents the efficiency of conversion of electrical energy to mechanical energy and vice versa. A high dielectric constant leads to better electrical impedance matching with the system electronics especially for small element phased array transducers. PZT ceramics have a typical k33 value of 0.70, but even higher coupling constants are preferred because they would increase not only the transmit and receive efficiency but also the bandwidth of the transducer. The recent discovery of high coupling in lead-based single crystal materials have generated a lot of interest in this regard.
Lead-based ferroelectric single crystals with the general formula Pb(Bxe2x80x2Bxe2x80x3)O3 where Bxe2x80x2=Mg2+, Zn2+, Sc3+ . . . and Bxe2x80x3=Nb5+, Ta5+ . . . , and the solid solution of these compounds with PbTiO3 have been shown to exhibit excellent electromechanical properties near the morphotropic phase boundary (MPB), the boundary separating the rhombohedral phase (spontaneous polarization along  less than 111 greater than ) and the tetragonal phase (polarization along  less than 001 greater than ). Some of the important compounds include Pb(Mg1/3Nb1/2l3)O3xe2x80x94PbTiO3 (xe2x80x9cPMNxe2x80x94PTxe2x80x9d), Pb(Zn1/3Nb2/3)O3xe2x80x94PbTiO3 (xe2x80x9cPZNxe2x80x94PTxe2x80x9d), and Pb(Sc1/3Nb2/3)O3xe2x80x94PbTiO3 (xe2x80x9cPSNxe2x80x94PTxe2x80x9d). FIG. 1 shows the phase diagram of PZNxe2x80x94PT and PMNxe2x80x94PT.
The electromechanical properties of PZN-9%PT (ratio of PZN to PT is xcx9c10 to 1) single crystals were first reported by Yonezawa et al. in 1969 (J. Jpn. Soc. Powder Metallurgy, 16, 253-258 (1969)), then by Kuwata et al. in 1982 (Ferroelectrics, 37, 579-582 (1981); Jpn. J. Appl. Phys. 21, 1298-1302(1982)). The high coupling constants of these single crystals make them attractive for transducer and actuator applications. These crystals with a composition on the rhombohedral side of the MPB and cut with the thickness along the [001] direction showed very high coupling (k33 greater than 0.92) and piezoelectric constants (d33 greater than 1500 pC/N). U.S. Pat. Nos. 5,295,487, 5,402,791, and 5,998,910 describe PZNxe2x80x94PT and PMNxe2x80x94PT systems at various compositions for ultrasonic transducer applications, the contents of which are hereby incorporated in their entirety. Coupling constants for slivers (k33xe2x80x2=0.82) and for bars (k33=0.92) have been reported for these systems.
Previous work has focused primarily on the  less than 001 greater than  longitudinal orientation, and the properties, especially the dielectric properties were found to be unstable near the MPB compositions because the compositions undergo a phase transition from rhombohedral to tetragonal phase. While the  less than 111 greater than  orientation has also been investigated for bar-shaped elements, the electromechanical properties along this orientation have been shown to be inferior to those along the  less than 001 greater than  orientation. For example, U.S. Pat. No. 5,998,910 and Kuwata et al. in Jap. J. Appl. Phys. 21 1298-1302 (1982) report low k33 values of only about 0.35-0.68 along the  less than 111 greater than  direction. Thus, in their materials the electromechanical properties appear to be very sensitive to the orientation and chemical composition of the crystal.
In addition, previous work disclosed single crystal materials primarily having a bar shape. Very little research have been conducted to understand the effect of the width orientation involving quasi-one dimensional structures such as slivers (see FIG. 2). The results were obtained for slivers that were cut with the thickness and the width along the  less than 001 greater than  orientation (see, for example, U.S. Pat. No. 5,402,791). However, slivers cut at the  less than 001 greater than  orientation showed the presence of spurious resonance in the frequency range of interest (see Lopath et al., Proceeding of the Tenth IEEE International Symposium on Applications of Ferroelectrics, East Brunswick, Aug. 18-21, 1996, pp. 543-546). For one dimensional (1-D) and 1.5-D transducer applications, it is critical to discover the best combination of both thickness and width orientation cuts to optimize the electromechanical properties of 1 -D or quasi-one dimensional structures such as slivers.
Medical ultrasonic imaging applications cover a wide frequency range spanning 1.5-40MHz depending on the organs to be imaged. The frequency depends on the thickness and sound velocity of the piezoelectric and the matching layer materials (f=v/2t, where f is the transducer frequency, v is the ultrasonic velocity of the piezoelectric material, and t is the material thickness). Thus, high frequency linear array applications at 8-15 MHz require a thickness of only about 130-200 xcexcm for PZT ceramic materials. PMNxe2x80x94PT or PZNxe2x80x94PT single crystals having a  less than 001 greater than  orientation have a lower velocity than PZT-type ceramics, and thus PMNxe2x80x94PT or PZNxe2x80x94PT wafers have to be even thinner (about 100 xcexcm). Because linear arrays have a typical wafer dimension of 40 mmxc3x974 mmxc3x970.2 mm, manufacturing these extremely thin crystals is very difficult because the crystals"" thinness makes them mechanically fragile. For high frequency imaging applications involving eye, intracardiac, and intravascular imaging, the wafer thickness drops to 20-30 xcexcm. At this thickness range, mechanical processing of  less than 001 greater than  orientated single crystals is even more challenging.
Recently, other workers have disclosed PMNxe2x80x94PT and PZNxe2x80x94PT oriented polycrystalline materials for use as ultrasonic transducers. (See Gentilman et al., xe2x80x9cProcessing and Application of Solid State Converted High Strain Undersea Transmitter Materials,xe2x80x9d paper presented at the Piezocrystals Workshop, Arlington, Va., Jan. 18-20, 2000). They report only on the  less than 001 greater than  orientation. No  less than 011 greater than  or  less than 111 greater than  oriented polycrystalline materials are disclosed.
This invention includes a transducer comprising a lead-based single crystal wherein the longitudinal or thickness direction of the crystal is diagonally oriented and has an effective coupling constant of at least 0.70. In one embodiment, the crystal is a face diagonally oriented crystal. Alternatively, the crystal maybe a body diagonally oriented crystal. The longitudinal or thickness direction of the cut may be from about 0 to about 20 degrees from the diagonal orientation.
Preferably, the lead-based crystal is of the formula Pb(Bxe2x80x2Bxe2x80x3)O3xe2x80x94PbTiO3 wherein Bxe2x80x2 can be at least one of the following: Mg2+, Ni2+, Sc3+, Yb3+, Fe3+, Mn3+, In3+, Ir3+, Co3+ or Zn2+, and Bxe2x80x3 can be at least one of the following: Nb5+, Ta5+, Te6+ or W6+. The lead-based crystal may further comprise one or more additional metal or metal oxides wherein the metal is Ba, Bi, Ca, Sr, La or Pt. In one embodiment, the lead-based crystal is of the formula Pb(Bxe2x80x2Bxe2x80x3)O3xe2x80x94PbTiO3 where Bxe2x80x2 is Mg2+, Zn2+, Sc3+ and Bxe2x80x3 is Nb5+. Specifically, the lead-based crystal is of the formula Pb(Mg1/3Nb2/3)O3xe2x80x94PbTiO3, Pb(Zn1/3Nb2/3)O3xe2x80x94PbTiO3, or Pb(Sc1/3Nb2/3)O3xe2x80x94PbTiO3. The molar ratio of Pb(Mg1/3Nb2/3)O3 to PbTiO3 may be from about 10:1 to about 1:1, or from about 6:1 to about 3:2 or from about 3:1 to about 5:3. For the lead-based crystal of the formula Pb(Zn1/3Nb2/3)O3xe2x80x94PbTiO3 and the ratio of Pb(Zn1/3Nb2/3)O3 to PbTiO3 may be from about 50:1 to about 2:1; or from about 25:1 to about 6:1; or about 15:1 to about 8:1.
Preferably, the crystal has an effective coupling constant of at least 0.80, more preferably of at least 0.85.
The invention also includes a lead-based single crystal wherein the longitudinal or thickness direction of the crystal is diagonally oriented and has an effective coupling constant of at least 0.70 wherein the ratio of the crystal length to thickness to width is from about (300 to 15):(5 to 1):(5 to 1). Preferably, the ratio of the crystal length to thickness to width is from about (150 to 10):(3 to 1):(3 to 1). More preferably, the ratio of the crystal length to thickness to width is from about (100 to 10):(3 to 2):(2 to 1).
In the face diagonal orientated crystals, the crystal has a width orientation of about 35 to 90 degrees, preferably of about 45 to 80 degrees, more preferably about 50 to 70 degrees away from the  less than 011 greater than  width orientation. In the body diagonal oriented crystal, the crystal has a width orientation of about xc2x110 degrees from the  less than 011 greater than  width orientation.
In an alternative embodiment, the transducer may comprise a lead-based single crystal orientated slightly off the  less than 001 greater than  orientation wherein the longitudinal or thickness direction of the sample is cut from 2 to about 20 degrees off the  less than 001 greater than  orientation and the coupling constant is greater than 0.75. Alternatively, the sample is cut from 2 to about 15 degrees (alternatively, 2 to 10 or 2 to 5 degrees) off the  less than 001 greater than  orientation and the coupling constant is greater than 0.80.
In another embodiment, a lead-based single crystal orientated in about the  less than 001 greater than t/ less than 010 greater than w orientation wherein the width orientation of the sample is cut from 2 to about 15 degrees (alternatively, 2 to 10 or 2 to 5 degrees) off the  less than 010 greater than  axis and the ratio the crystal length to thickness to width is from about (300 to 15):(5 to 1):(5 to 1). Also, the sample may be cut from 15 to about 25 degrees off the  less than 010 greater than  width orientation and the ratio of the crystal length to thickness to width is from (300 to 15):(5 to 3):(2 to 1).
The invention also includes a transducer comprising a plurality of lead-based single crystal transducer elements. Moreover, the lead-based single crystal materials may be embedded in a polymer to form a single crystal/polymer composite. It may be an ultrasonic transducer comprising one or more piezoelectric components that function as transmitting and/or receiving elements; and electrodes placed upon opposite surfaces of the elements, and wherein each lead based piezoelectric component is diagonally oriented and has an effective coupling constant of at least 0.70. In addition, for sliver elements, the ratio of the crystal length to thickness to width is from about (300 to 15):(5 to 1):(5 to 1). For bar elements, the ratio of crystal length to width is (100 to 5):(5 to 1).
In another alternative embodiment the invention includes a transducer comprising lead-based diagonally oriented polycrystalline material. In the polycrystalline material the effective coupling constant may be greater than 0.70, preferably greater than 0.80, more preferably greater than 0.85.